A question for any scientists out there

[idonthaveaname]

Re: A question for any scientists out there

"The pressure decreases when the boat is put into the water."

Not to start an argument, but I don't believe that was the question.

It was asked after you inflate the tubes to the proper air pressure, then add the engine, put it in the water and add the passengers, does the air pressure in the tubes increase... the correct answer is yes the pressure will increase.

 

[NetDoc]

Re: A question for any scientists out there

The pressure decreases when the boat is put into the water. That's it. No if's, and's or but's.

This is just wrong. It actually depends on the temperature of the boat and the water. If the water is cooler, STP gas laws teach us that the pressure will decrease, but if the water is warmer, then the opposite will occur. This is rare, but its an obvious "if, and or but". The difference is relational to the absolute pressure Say the temp of the air is 70 degrees and the water temp is 65. In order to calculate pressure change we have to convert to Rankine (+459.67) which we will round to 460 for this. Then we divide the final by the start pressure and multiply by 100. 525/530*100=99.05%. The final outcome will be about 1% less for every five degrees of temperature change. Not much really. It makes a bigger difference if you have a hot fill (like a scuba tank) and then submerge it into the ocean. Its simple chemistry/physics here and anyone who has studied STP gas laws will follow right along. I hope this helps.

 

[1980Coronado]

Re: A question for any scientists out there

Every 33 ft of salt water yields 14.7 psi. That's less than a half psi per foot. What is your boat's draft? Now figure just how much of the entire boat (%) is beneath the water line? Get the idea? The increase is incredibly minimal. Welcome to Scuba 101 as I like to tell my students: Its not rocket science, its submarine science!

You're confusing the pressure exerted on a submerged body by the weight of the water, with displacement. When a body is floating freely it displaces a sufficient volume of fluid to just balance it's own weight. It is not submerged so therefore there is no water above it to exert a pressure. If the inflatable were a ridged body, the pressure inside would not increase at all when loaded/floated or submerged. For example, when you submerge a scuba tank the pressure inside does not increase or decrease because there is no change in volume. However, because the inflatable material is flexible it compresses under it's own weight when floated and then even more as it's loaded, therefore reducing the volume inside the inflatable chamber and increasing pressure. When you take Temperature into account Boyle's Law PV=nRT.... or P1V1/T1=P2V2/T2, the cooler water will decrease pressure. So the net effect in pressure may actually be relatively small and could go either up or down depending on the magnitude of the temperature change or amount of weight in the inflatable.

 

[1980Coronado]

Re: A question for any scientists out there

Here's something to consider: what PSI are you putting in your boat? The more air you add, the denser the boat and actually the less it floats. A normal Scuba tank has about an 8 pound swing from full to empty. If we don't accommodate for that swing we find it hard to do our safety stop!

For Scuba divers the most important thing about Boyles law can be summed up like this: Breathe Or Your Lungs Explode Stupid.

Apples and Oranges.......an 8 pound swing in a body that starts neutral to slightly negatively buoyant doesn't compare to something that has an order of magnitude more positive buoyancy. The weight of the air in the inflatable is negligible and measuring the difference in weight for a couple psi would be all but impossible.

Comparing the inflatable to a divers lungs is also apples and oranges....you don't hold your breath because as you surface the air in your lungs expands due to the pressure difference. This is why you consume more air at depth than when on the surface. The pressure effect on the inflatable due to the weight of the water is zero because it's not submerged.....the water being displaced isn't in any way related to the pressure inside the inflatables chambers, it's a function of weight and area.

 

[jacoboregon]

Re: A question for any scientists out there

I wonder if the "scientists" own and use inflatable boats?

tohatsu guru is absolutely correct. The water will ALWAYS be cooler than the ambient air temp and the pressure WILL decrease. (Unless you are operating in arctic regions, at which the temp differences will have a negligible effect anyways!)

AND ONCE AGAIN! If the patch was done properly it WILL be strong enough that the material around it will give before the patch does.

You are trying to strain out a gnat.

What you need to be concerned with is: DID YOU USE CORRECT TECHNIQUE/MATERIAL IN DOING THE PATCH? ... If so, ALL the other "scientific" stuff does not matter.

 

[idonthaveaname]

Re: A question for any scientists out there

beer and popcorn, anyone?

 

[1980Coronado]

Re: A question for any scientists out there

beer and popcorn, anyone?

Cold and free is my brand

 

[1980Coronado]

Re: A question for any scientists out there

I wonder if the "scientists" own and use inflatable boats?

tohatsu guru is absolutely correct. The water will ALWAYS be cooler than the ambient air temp and the pressure WILL decrease. (Unless you are operating in arctic regions, at which the temp differences will have a negligible effect anyways!)

AND ONCE AGAIN! If the patch was done properly it WILL be strong enough that the material around it will give before the patch does.

You are trying to strain out a gnat.

What you need to be concerned with is: DID YOU USE CORRECT TECHNIQUE/MATERIAL IN DOING THE PATCH? ... If so, ALL the other "scientific" stuff does not matter.

Once you fill it with air and put it in the water at some point the system will achieve equilibrium at some pressure and temp...after that the pressure will increase when you load it...but it's won't be by much.

Bottom line is the patches are designed to hold when applied properly.

 

[NetDoc]

Re: A question for any scientists out there

You're confusing the pressure exerted on a submerged body by the weight of the water, with displacement.

I'm not the one confused here. Nothing floats perfectly on the surface. Once part of the vessel descends below the plane we call the surface, pressures start to act on it. What ever part of the vessel is below the surface is submerged. As I pointed out, its only about 1/2 psi/foot so the increase is "incredibly minimal". You might not fully understand the mechanism or science, but that doesn't make it so. When you are dealing with a non rigid vessel, you should really look at this as a hybrid system when trying to describe it. Once you do, you can mathematically determine that the increase is indeed "incredibly minimal".

For example, when you submerge a scuba tank the pressure inside does not increase or decrease because there is no change in volume.

Absolute pressure does not change, however relative (usable) pressure does. However, this is NOT a rigid bodied vessel so pressure does affect its volume as well as its buoyancy. If you think that there is very little weight take a tube into a pool and see how deep you can get before the weight of the water on your chest interferes with your ability to breathe. There's a reason snorkels are only 18" long.

When you take Temperature into account Boyle's Law PV=nRT.... or P1V1/T1=P2V2/T2,

You are combining Boyles with Charles' and Gay-Lussac's law. Boyle's Law only deals with volume and pressure, while Charles' Law deals with Volume and temperature and Gay-Lussac's deals with pressure and temperature. Together they form the ideal gas laws and the biggest noob mistake is to not account for absolute pressure and temperature. For temperature add 460 F (273 C) to get to Rankine and for pressure add 14.7 psi (1 bar) for atmospheric.

The water will ALWAYS be cooler than the ambient air temp and the pressure WILL decrease. (Unless you are operating in arctic regions, at which the temp differences will have a negligible effect anyways!)

Or unless you live in Florida. Our springs put out 68-72 degree water all year long. That's why manatees migrate into the springs every winter. Last I checked, we are more sub tropic than arctic.

I have also seen water temps here in the Keys outperform air temps in the winter. The coldest water I have seen down here has been 68 F, and yet I dove when the air temps were a stoopidly cool 42F last year.

The science is not that hard and it clears all the apocryphal BS that people are tossing out like they know better. Like I said, its not rocket science: its submarine science and that's where I tend to play (and instruct).

 

[jacoboregon]

Re: A question for any scientists out there

.....

The science is not that hard and it clears all the apocryphal BS that people are tossing out like they know better. Like I said, its not rocket science: its submarine science and that's where I tend to play (and instruct).

Exactly.... we are talking inflatable boats. NOT submarines, or scuba tanks, or your lungs......

californiadreamin, you can totally disregard all the "scientific" crap on here and relax as long as the patch was properly applied. The rest of the stuff is totally inconsequential to your real question of whether or not your patch will hold. The only real science that applies to your situation is that of the bonding process of the patching.

Honestly, I'm not trying to step on any toes. I just want the op to have what information really applies to his situation. Hopefully we post stuff to help one another out, not to confuse the issue.

Enough said.

 

[Sea Rider]

Re: A question for any scientists out there

Amen!!!!

Happy Boating

 

[NetDoc]

Re: A question for any scientists out there

I just want the op to have what information really applies to his situation.

Which I gave him in post five. After that, I just want to debunk the pseudo-science. You might be OK with that going unchallenged... I am not.

 

[1980Coronado]

Re: A question for any scientists out there

Nothing floats perfectly on the surface. Once part of the vessel descends below the plane we call the surface, pressures start to act on it. What ever part of the vessel is below the surface is submerged.

The only force acting as a result of the weight of the water is on the sides/ends of the vessel, which is at most a couple of inches below the surface, there is no force exerted on the bottom due to the weight of the water, because it is displacing the water. Again...it's small...and not changing the volume enough to matter.

I agree I mixed Boyle with Charles (it's been awhile)...Boyle assumes an isothermal process. The point is the same...temperature affects volume, which affects pressure.

Absolute pressure does not change, however relative (usable) pressure does.

Only when you breath it through the regulator, which is compensating for the differential.

However, this is NOT a rigid bodied vessel so pressure does affect its volume as well as its buoyancy.

By an infinitesimally small amount. They are fairly rigid...but not like a metal tank.

 

[TOHATSU GURU]

Re: A question for any scientists out there

This is just wrong. It actually depends on the temperature of the boat and the water. If the water is cooler, STP gas laws teach us that the pressure will decrease, but if the water is warmer, then the opposite will occur. This is rare, but its an obvious "if, and or but". The difference is relational to the absolute pressure Say the temp of the air is 70 degrees and the water temp is 65. In order to calculate pressure change we have to convert to Rankine (+459.67) which we will round to 460 for this. Then we divide the final by the start pressure and multiply by 100. 525/530*100=99.05%. The final outcome will be about 1% less for every five degrees of temperature change. Not much really. It makes a bigger difference if you have a hot fill (like a scuba tank) and then submerge it into the ocean. Its simple chemistry/physics here and anyone who has studied STP gas laws will follow right along. I hope this helps.

Pete,

I'm in Florida. I am surrounded by some 40 new inflatables and by about 20 that are in for repair and all of them had/have/will have to have the pressure topped off after they have been put into the water. Are there circumstances where that might not be true? Yes. But, we are talking about "normal" and normally it's going to be hotter out of the water than in it. This isn't an opinion. It's a demonstrable fact. Simply take your inflatable outside at X time and measure the pressure in the tubes. Then put it into the water, wait 15 minutes and then measure it again. There will be a loss. How much of a loss will vary. As the day heats up that loss may eventually disappear.

 

[NetDoc]

Re: A question for any scientists out there

Are there circumstances where that might not be true?

Which, by definition, is a long, long ways from "ALWAYS". People who deal in absolutes are almost always wrong. But hey, you can argue with the science all you want. There will be about a 1% decrease in absolute pressure for every five degree change in temperature Fahrenheit. That's it. That's the science. Argue with it all you want.

BTW, absolute pressure for a 3psi boat fill is actually 17.7psi. The former is relative to atmospheric (ambient) pressure. So, it would take almost a 30 degree temperature change to reduce the boat pressure 1 psi. Rly.

 

[CapeAnn]

Re: A question for any scientists out there

At the end of the day - all the variables (and there are many) aren't biggies. The biggest variable /lynchpin is the patch job.

To sum up everything I will leave posters with this formula - P1V1/T1 = P2V2/T2 T = temperature. Additionally, if we are looking at forces using vectors helps. However, if we're being practical - it's mostly for navel contemplation. Not naval. Bad pun.

For me - it's been a fun thread. I can't drink beer so I am going to have some Florida orange juice. Digestion being mostly a chemical reaction.

 

[Sea Rider]

Re: A question for any scientists out there

I'm not good at math, but you must have in consideration that for any inflatable inflated at 3.0 psi at shade if you like and placed on water, tubes's pressure will increase/decrease accordingly depending on atmospheric presure & height, water temp and ambient temp, as you can have warm water in warm environments and cold water on warm environments. Assume this issue will vary from country to country, definitely ain't rocket science.

Happy Boating

 

[CapeAnn]

Re: A question for any scientists out there

I wonder if the "scientists" own and use inflatable boats?

.. If so, ALL the other "scientific" stuff does not matter.

I'm a former scientist. I used to do some quantitative work. I dare you to knock my retractable pencil off my shoulder. Dare ya'!

Cheers,

CapeAnn

 

[TOHATSU GURU]

Re: A question for any scientists out there

And your still missing the point. Let's say that your boat is filled to 3.5 PSI. You drop it in the water and 30 minutes later your at 3.3 PSI. Is there a drop in pressure? Yes. Is it significant? On a rib, no. On an air deck, yes. Going back to the original question: Yes, there is a pressure drop.

Your science is pointless...At this point I'm talking the reality of what actually happens to an inflatable boat under most circumstances. I simply don't care about the formula or who formulated it. Rather than argue with me...Simply pressure test a boat, drop it in the water and verify what happens. Then, if you want to prove how smart you are, state the formula that accounts for people on the internet arguing with experts

And I'm not picking a fight with you...Actually I really wish you would verify what I've told you so that you will be a little less "absolute" yourself. By the way, where do you dive out of?

 

[NetDoc]

Re: A question for any scientists out there

I simply don't care about the formula or who formulated it.

Enough said right there! Apparently, the OP was more interested in the science of this than you are. I'll take this as a quaint way of you saying "My mind is made up, so please don't confuse me with the facts!"

Eons ago man started trying to make sense of the physical world we all live in. Since then, dogmatic people who don't care about the formula have been convinced that their interpretation of certain phenomenon is fact and that everyone else is wrong. Fortunately, the scientists prevailed and so we have more than four elements, the earth is no longer flat and our comprehension of the physics and chemistry of our world is amazingly complete. The science is simple enough that most anyone could put their mind to comprehension IF (and that's a "big if") they wanted to. If you have any questions about the science of what is happening here, I would gladly explain it. I still stand by my post #5: the effects would be minimal.

As for my background, I first started out as a Chemistry major for the University of Florida. Although I changed it to Russian, I still worked my way through college as a lab tech/machinist in a P-Chem lab, building and operating a super conducting magnet. We did spectral analysis on various states of hydrogen and we routinely got down to 10[SUP]-5[/SUP] torr and 1.5 C. I have a solid grasp of how temps, pressures and volumes can affect things.